Diffusion of water in rhyolite glass: diffusion–reaction model

Experimental measurements of diffusion profiles for water in rhyolite glass (obsidian) from previous publications [J.R. Delaney, J.L. Karsten, Earth and Planet Sci. Lett. 52 (1981) 191; J.L. Karsten, J.R. Holloway, J.R. Delaney, Earth and Planet Sci. Lett. 59 (1982) 420; K.E. Lapham, J.R. Holloway, J.R. Delaney, J. Non-Cryst. Solids 67 (1984) 179; Y. Zhang, E.M. Stolper, G.J. Wasserburg, Geochim. Acta, 55 (1991) 441; Y. Zhang, E.M. Stolper, P.D. Ihinger, Am. Min. 80 (1995) 593] are compared with calculated profiles from a diffusion–reaction model. Good agreement between experiments and the model is found. In this model dissolved molecular water diffuses in the glass and reacts with the glass network to form immobile OH groups. Diffusion coefficients for molecular water have an activation energy of about 84 kJ/mol from 400°C to 950°C, for both local equilibrium and non-equilibrium. Profiles of OH groups and of molecular water fit those predicted by the model.